Primary Molybdenum and many Copper mines contain molybdenum (Mo) sulfides and also iron and copper sulfides in their ore bodies. Typically, a range of copper sulfides, iron sulfides, molybdenite and other less prevalent sulfides are associated in the ore bodies. In order to achieve an economic value, the Molybdenite (MoS2) must be separated and concentrated to approximately 45-50% Mo. This is typically done by a series of flotation cells, wherein various chemical treatments result in floating of molybdenite while copper-containing and iron-containing minerals do not attach to a gas bubble and therefore do not float. Such flotation separation technologies are well known in the art, and an early reference on various chemicals which enhance the separation of molybdenite from other sulfides is U.S. Pat. No. 2,608,298. Chemical depressants are used to accomplish this separation in a flotation process after an initial or bulk concentrate is produced. Added reagents depress the other sulfides by inhibiting their flotation, allowing primarily just molybdenite to attach to the gas bubble and being recovered from a froth. Typically, molybdenite flotation and recovery from bulk copper molybdenum concentrates has involved the use of alkalisulfides, Nokes reagents, cyanides, oxidants, and/or thermal treatment to depress copper and iron sulfide minerals. The bulk copper/molybdenum concentrate or molydenum plant feed generally varies from 0.2% Mo to 3.5% Mo, and with traditional reagents the single-stage rougher flotation recovery of molybdenite varies between 40% and 90+% at a concentrate grade of 5% to 25% Mo.
U.S. Pat. No. 2,608,298 teaches using a mixture of polysulfides and thiosulfate, in combination with at least one water soluble inorganic metal salt, other than salts of alkali and alkaline earth metals. Exemplary salts include ferric, ferrous, copper, zinc, or aluminum. The patent teaches that these salts can be generated by adding acid to the slurry to reduce the pH below 7, but not below about 5.5. Subsequent addition of the polysulfide and thiosulfate reagent is taught to raise the pH and in some cases to bring the pH “back to a point above the neutral point.” This patent teaches that a substantial proportion of each of thiosulfate and polysulfide must be present, and that there is little advantage gained by using more than 8 pounds of reagent per ton.
Current practices in the industry consist of a number of different chemical schemes which use Sodium hydrosulfide (NaHS or NaSH), Ferro Cyanide, or Nokes reagent (Blend of thiophosphates or dithioarsenates and usually also containing sulfides) conditioning followed by flotation and in some cases, additional grinding, to achieve a marketable MoS2 concentrate. Sodium hydrosulfide (NaHS) flotation is the benchmark standard, in very common use, but with inherent HSE (Health, Safety and Environmental) concerns, and also readily releasing toxic H2S when pH is reduced. An option for lower pH separation is using Ferro-Cyanide where the process consists of conditioning a Cu/Mo concentrate with a Potassium Ferro Cyanide at an acidic pH followed by multiple steps of flotation and possible use of additional depressants such as alkaline dithiophosphates to make a final molybdenite concentrate. This process also has significant health and safety issues, and is generally less effective in comparison to using NaHS.
The Nokes depression scheme described in U.S. Pat. No. 2,811,255 consists of conditioning a slurry comprising copper-containing and molybdenum-containing minerals, (“a CuMo slurry”) with Nokes Reagent (Sodium Dithiophosphate or Sodium Dithioarsenate) followed by multiple steps of flotation until an acceptable Molybdenite concentrate is achieved.
Other processes have been used to depress the copper and iron sulfides and allow the MoS2 to be concentrated by flotation. A partial list includes: 1) Cu/Mo Concentrate roasting followed by moly flotation as described by Inspiration Cons Copper Co, Kennecott Copper Corp, Hayden Division and others was used in the 1960's; 2) Autoclaving in addition to Nokes Reagent in moly flotation as described by Inspiration Cons Copper Co, 1969; 3) Ferro-Cyanide conditioning and moly flotation as used by FMI Morenci Division; 4) Open steaming in conjunction with Nokes Reagent and moly flotation as described by Bagdad Copper Co, 1970; 5) Hydrogen Peroxide conditioning and moly flotation as described by Magma Copper Co, San Manuel Moly Plant, circa 1970's; 6) Hot water dilution in conjunction with NaHS conditioning and moly flotation as described by Anamax Copper Co, 1978; 7) Conditioning with NaHS in conjunction with moly flotation at monitored ORP ranges using Nitrogen for optimum depressant consumption as used by FMI Sierrita and Bagdad Divisions and others; 8) ozone conditioning as described by Ye, W. H. Jang, M. R. Yalamanchili, 1990; and 9) Molybdenite flotation using sodium sulfide while adjusting the reducing potential as described by M. Kolandoozani and H. Noon.
WO 99/66013 discloses the formation of a compound of molybdenum and sulfur using a molybdenum and thiuram disulfide in an organic solvent, and the removal of solvent. Calcium polysulfide (“CaPS”) is used in certain hydrometallurgical applications, such as to fixate chrome and to remove more soluble heavy metals from wastewater streams.
Existing flotation practice for the separation of molybdenite from iron/molybdenum and/or copper/molybdenum concentrates generally utilizes types of reagents that lead to concern with respect to health, safety, and environmental issues. There are many such concerns, including transporting materials that contain or readily form hydrogen sulfide, utilizing reagents in the flotation process that are highly toxic and/or that in use form highly toxic hydrogen sulfide off-gassing, and other concerns. With some known reagents it may result in low Mo recovery or requires many stages of flotation. Any improvements in molybdenite flotation practice, especially with respect to health and safety, would be of significant importance.